It's possible that it's sluggish because it's indexing books. Do a search for some random string (eg "aaaaaaaa"). If it says "No results found", it's not indexing, but it if says "XXX books not yet indexed", it is. If you move to the line that says that with the 5-way controller, and press "Select", it will show you what the books are that aren't indexed. Sometimes books do get "stuck" while indexing, in which case deleting and re-adding them generally fixes the problem.